Gilles had an impressive first half of the season with Arsenal Reserves, making 9 appearances and scoring four goals.[2] He made his first team debut for the club when he started a League Cup match against West Bromwich Albion on 22 September 2009, playing 58 minutes before being substituted for Carlos Vela.[3] On 9 December 2009 Sunu made his Champions League debut in Arsenal's 1–0 away defeat to Olympiakos, coming on for Jack Wilshere on 76 minutes.[4]

On 18 February 2010, Sunu signed for Derby County on loan until the end of the 2009/10 season[5] Gilles made his debut for The Rams as a first-half substitute in a 1–0 defeat to Swansea City, coming on for the injured Kris Commons before being later substituted on the 85-minute. Sunu scored his first goal in a 4–1 defeat away to Reading on 10 March 2010. After Stephen Pearson and Rob Hulse worked a smart one-two on the left side of the area and when Pearson was halted by a challenge, Sunu poked the loose ball past Adam Federici from the edge of the area.[6] In total, Sunu made nine appearances for Derby before returning to Arsenal slightly earlier than intended after picking up an injury in Derby's penultimate game of the season away to Bristol City.[7]

On 31 January 2011, Gilles Sunu opted to join FC Lorient on loan for the remainder of the 2010–11 campaign.[9] He made his debut for Les Merlus as a second-half substitute in the 2–0 victory over SM Caen.[10] Gilles got his first assist in the 3–2 away victory to RC Lens. After rounding Lens goalkeeper Vedran Runje, Sunu flicked the ball back from the byline for Kevin Gameiro to head in from 5 yards to make the score 2–2.[11] Gilles completed his loan at Lorient, making 9 appearances all from the substitutes bench.[12] Sky Sports reported that Lorient manager Christian Gourcuff was keen to re-sign Sunu as well as his Arsenal team mate Francis Coquelin on an additional years loan, with Sunu hinting that he would welcome a return to the seaport town.[13]

Gilles Sunu signed a 4-year contract with Lorient on 31 August 2011 just three days after being an unused sub in Arsenal's 8–2 defeat to Manchester United.[14] Sunu made his full debut for Lorient in the 1–1 away draw against FC Sochaux, coming on as a 62nd minute substitute for Arnold Mvuemba.[15] On 6 November 2011, Sunu scored his first goal for Lorient and made an assist in the 2–0 home win against AC Ajaccio. Sunu went past his man and cut the ball back for Yann Jouffre to score from outside the box. Before Jouffre turned provider and played Sunu in on goal with only the keeper to beat.[16] Sunu scored his second goal of the season in the quarter finals of the Coupe De La Ligue in the 1–0 away victory to Le Mans FC. Gilles beat Le Mans 'keeper Giorgi Makaridze at the near post on the 70th minute after coming on as a 62nd minute sub for Innocent Emeghara.[17]